1. Field of the Invention
This invention relates to a process for the preparation of polyisocyanates containing isocyanurate groups by trimerizing a proportion of the isocyanate groups of organic polyisocyanates using novel trimerization catalysts and terminating of the trimerization reaction by the addition of a catalyst poison, and the use of the products obtained by the process according to the invention, optionally freed from monomeric starting polyisocyanate and/or blocked with blocking agents for isocyanate groups, as an isocyanate component in the production of polyurethanes.
2. Description of the Prior Art
Processes for the trimerization of organic isocyanates, in particular polyisocyanates, are known in large numbers (J. H. Saunders and K. C. Frisch, Polyurethanes Chemistry and Technology, page 94 et seq (1962)). Strong organic bases are suitable as catalysts for trimerization, e.g. those metal salts of carboxylic acids which are alkaline in action, metal alcoholates, metal phenolates, alkali metal carbonates, tertiary amines, tertiary phosphines and the "onium" compounds of nitrogen and phosphorus as well as basic heterocyclic compounds of these elements. The catalysts are frequently used in combination or together with other cocatalysts such as mono-N-substituted carbamic acid esters (A. Farkes and G. A. Mills, Advances in Catalysis, Volume 13, 393 (1962)). Elaborate catalyst systems are in most cases used since it is known that simple metal salts such as carboxylates or alcoholates are only capable of effecting cyclotrimerization of isocyanates if used at relatively high concentrations and at high temperatures (see e.g. British Pat. No. 809,809, Example 6).
If the trimerization with metal salts is to be carried out in a solvent, it is necessary to use highly polar aprotic solvents such as dimethyl formamide or dimethyl sulphoxide since only these are capable of dissolving inorganic metal salts and metal salts with a small organic group (German Offenlegungsschrift No. 2,839,084). Even then, catalyst concentrations of from 0.1 to 0.5% by weight are required. This also applies if the solvents used are protic solvents but react with the isocyanate to form urethanes and thus lower the isocyanate content or form precipitates and cloudiness so that the reaction product must be filtered (British Pat. No. 920,080).
Furthermore, the metal salts used in the state of the art effect rapid trimerization only in the case of aromatic isocyanates while aliphatic mono and polyisocyanates require a high catalyst concentration and comparatively high temperatures which frequently result in an uneven exothermic reaction, and in the case of polyisocyanates lead to the formation of highly viscous, strongly discolored products (see U.S. Pat. No. 3,330,828, Examples 1 to 4; British Pat. No. 952,931, Example 3; German Auslegeschrift No. 1,013,869, Example 3) or the formation of gel particles (British Pat. No. 966,388, Example 3) with the result that the products are not suited as isocyanate components for high quality polyurethane lacquers. One serious disadvantage of metal salt catalysis is also to be seen in the fact that when catalysis is stopped, e.g. by the addition of acid compounds, inorganic salts are formed which are insoluble in the polyisocyanate and cause cloudiness. In the more recent processes of the state of the art, therefore, special organic bases are used as trimerization catalysts. Thus, for example, Mannich bases (German Offenlegungschrift No. 2,551,634 and German Offenlegungsschrift No. 2,641,380) or tertiary phosphines are used for the trimerization of aromatic polyisocyanates. In the case of phosphines, uretdiones are first formed which then react to form the isocyanurate in a second reaction phase (German Offenlegungsschrift No. 1,201,992). For the trimerization of (cyclo) aliphatic diisocyanates there have recently in many cases been used organic bases with a betaine structure such as quaternary ammonium hydroxides (European Application Nos. 010,589 and 009,694), aminimides (J. E. Kresta, R. J. Chang, S. Kathiriya and K. C. Frisch, Makromol. Chem. 180, 1081 (1979)) and aziridine derivatives in combination with tertiary amines (German Auslegeschrift No. 2,325,826).
All of these catalyst systems have the disadvantage that quite specific temperature intervals must be observed and in some cases the process must be carried out solvent free. In others, the process may only be conducted in selected solvents, and in still others trimerization may be carried out either on aromatic polyisocyanates alone or on aliphatic polyisocyanates alone.
It is therefore an object of the present invention to provide a process by which colorless polyisocyanates containing isocyanurate groups, both those with an aromatic and those with an aliphatic structure, may be prepared by a technically simple procedure with one and the same catalyst, either with or without solvent and without elaborate temperature control.
This problem was surprisingly able to be solved by the process according to the invention described below, in which novel trimerization catalysts are used.